RGB SCART - ARGHHH

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GadgetUK164
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Re: RGB SCART - ARGHHH

Post by GadgetUK164 » Sun Jun 28, 2020 2:06 pm

I've sorted it! Then I found there were jail bars - sorted that too (well, reduced it by like 95%)
a.png
a.png (312.84 KiB) Viewed 694 times
Looks crystal clear now =D

I am editing the video at the moment. I will post what I needed to do here later =D
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exxos
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Re: RGB SCART - ARGHHH

Post by exxos » Sun Jun 28, 2020 2:10 pm

GadgetUK164 wrote:
Sun Jun 28, 2020 2:06 pm
I've sorted it! Then I found there were jail bars - sorted that too (well, reduced it by like 95%)
:bravo: What was it in the end then ?
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Re: RGB SCART - ARGHHH

Post by GadgetUK164 » Sun Jun 28, 2020 6:45 pm

exxos wrote:
Sun Jun 28, 2020 2:10 pm
GadgetUK164 wrote:
Sun Jun 28, 2020 2:06 pm
I've sorted it! Then I found there were jail bars - sorted that too (well, reduced it by like 95%)
:bravo: What was it in the end then ?
Currently this video isn't public - so please don't share it yet.



I basically had to fit a diode on the vsync signal (which was used for the RBG switching on pin 16), and a 47uF cap. In addition I found the jailbars on this cable were related to the composite level. Originally it had an 82 ohm resistor feeding the vsync signal, and an 82 ohm resistor feeding the composite signal (neither of these were right imo). I tried 150ohm on the composite and the jailbars improved, then 330 ohm and it was much much better (but not gone). I then jumped to 1K and it was pretty much perfect.

As mentioned in the video, I believe a diode should be used when the vsync is used on a SCART cable like this - otherwise you could kill the GLUE via the vsync signal being directly connected from the exposed SCART end, directly via the cable straight to the glue. Are all ST revisions like this? I haven't checked my main ST, but on this ST the Glue vsync goes straight to the AV port with nothing inbetween and no clamping that I can see. I guess the IC may have clamping internally though.
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Re: RGB SCART - ARGHHH

Post by exxos » Sun Jun 28, 2020 6:56 pm

ahh yep I think I've seen that mod done on some cables.
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Re: RGB SCART - ARGHHH

Post by GadgetUK164 » Sun Jun 28, 2020 7:10 pm

exxos wrote:
Sun Jun 28, 2020 6:56 pm
ahh yep I think I've seen that mod done on some cables.
On a seperate note - will you be creating any more of the PSU boards for the ST at all? I noticed they are out of stock on your website!
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Re: RGB SCART - ARGHHH

Post by exxos » Sun Jun 28, 2020 7:12 pm

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.

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GadgetUK164
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Re: RGB SCART - ARGHHH

Post by GadgetUK164 » Sun Jun 28, 2020 7:18 pm

exxos wrote:
Sun Jun 28, 2020 7:12 pm
See here viewtopic.php?f=90&t=2144
Thanks, will just keep an eye on the website then!
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tzok
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Re: RGB SCART - ARGHHH

Post by tzok » Sun Jun 28, 2020 9:53 pm

Diode and capacitor are needed, because this input has 75 ohm impedance on the TV side, so you can't use 12V out from ST and a resistor divider (actually from the ST you can, but not from STe*), and it needs to be about 2 VDC voltage. VSync obviously is not a DC, and some TVs can switch between composite and RGB really fast. However, some TVs will still not like it.

* - on the STe the 12V is fed into the Monitor port via the 4,7k Resistor, while on the ST(f) it is connected via the 1.2k resistor. The SCART selection/blanking input is high impedance, but the RGB mode selection input is 75 Ohm. So on STf it would result in 0.7V (what is below the spec, but should be enough to switch into RGB mode), but on STe it would be only 0.2V on RGB switch input.

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Re: RGB SCART - ARGHHH

Post by GadgetUK164 » Sun Jun 28, 2020 10:43 pm

tzok wrote:
Sun Jun 28, 2020 9:53 pm
Diode and capacitor are needed, because this input has 75 ohm impedance on the TV side, so you can't use 12V out from ST and a resistor divider (actually from the ST you can, but not from STe*), and it needs to be about 2 VDC voltage. VSync obviously is not a DC, and some TVs can switch between composite and RGB really fast. However, some TVs will still not like it.

* - on the STe the 12V is fed into the Monitor port via the 4,7k Resistor, while on the ST(f) it is connected via the 1.2k resistor. The SCART selection/blanking input is high impedance, but the RGB mode selection input is 75 Ohm. So on STf it would result in 0.7V (what is below the spec, but should be enough to switch into RGB mode), but on STe it would be only 0.2V on RGB switch input.
The 12v (via 1K2) on my couple of STFM here wasn't enough to switch into RGB (when I tried to reduce it) but as you say, if its high impedance that explains why I wasn't able to do that. But that 12v feed was sufficient to switch the TV onto the correct SCART input (via pin 8). I will test the cable later on the STe too - curious to see if it works OK there.
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tzok
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Re: RGB SCART - ARGHHH

Post by tzok » Wed Jul 01, 2020 11:51 pm

At the TV, pin 8 has a few kOhms, while blanking input (pin 16) is terminated with 75 Ohm resistor.

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